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ollypenrice

3 Sharpless nebulae in Vulpecula.

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Here we have Sh2-86, 88 and 90. In the upper right we see a few bright stars from the open cluster Stock One creeping in. The dark nebula (upper right) and rather attractive sea-horse-like emission nebula (lower left) have defeated me regarding identification. To get your bearings, M27 is not far off shot to the lower left and the Coathanger Cluster (Brocchi's) lies somewhat below this field.

The lower panel comes from last year, HaLRGB, and the upper from two nights ago (HaRGB), both taken with guest Paul Kummer.

Hardware: the house Tak FSQ106N dual rig with Atik 11000 mono CCD cameras riding on Mesu 200 mount. Processing: AstroArt, Pixinsight, Registar and mostly Photoshop CS3.

Taken last year and this with regular summertime guest Paul Kummer - using the term 'summertime' rather optimistically this year!

1767870240_SH286ETCWEB.thumb.jpg.49e6338c34b9a25b1a9bff4e66b500a6.jpg

Olly and Paul.

 

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Gorgeous Olly - what a busy bit of the sky you have there. Truly jam-packed!

Wondering how you removed the gradient on this one?!

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9 hours ago, Petergoodhew said:

Gorgeous Olly - what a busy bit of the sky you have there. Truly jam-packed!

Wondering how you removed the gradient on this one?!

Thanks Peter. I used ABE on the individual panels because there was too little sky for DBE. I also discovered from Warren Keller's book how to de-posterize the gradient maps made by PI. Right click on the image, choose Screen Transfer Functions and then Use 24 bit LUTs. (It's obvious, innit? Who says PI is obscure? Use something called 24 bit Look Up Table to deposterize the model. It's not as if they buried under a tab in ABE itself and called it Deposterize Background Model.)

Olly

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Lut is not only for background models, but any image that with the stf applied, shows posterisation. That's how I use it. It makes sense to put it in the stf button group. Btw, lut affects only the image on screen. The applied background model is smooth as is. With or without lut.

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2 hours ago, wimvb said:

Lut is not only for background models, but any image that with the stf applied, shows posterisation. That's how I use it. It makes sense to put it in the stf button group. Btw, lut affects only the image on screen. The applied background model is smooth as is. With or without lut.

That's odd, since Warren Keller suggests you use it. I did wonder how it would actually affect the image since STF is just a screen stretch, though. So I take your point.

However, when I applied a posterized DBE model on another image this week the posterization became visible on a hard stretch of the data. When I read up on this deposterization in the model and applied it, I could no longer see posterization in the highly stretched image. Of course this could just have been because the model wasn't the same one and was smoother for other reasons.

So if LUT is a waste of time, how do you deposterize the background model? And why do you suppose Warren recommends it?

Thanks in advance for any advice! 

Olly

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I'll do some more homework on this one. But what I've understood so far is that lut is a screen feature. However, if you have a 16 bit integer image, and stretch it hard in 16 bit, I would expect posterisation. But if the software converts the image to a higher bit depth, such as 32 bit floating point accuracy, there shouldn't be visible posterisation at any stretch. A 16 bit histogram looks spiked, while a 32 bit fp histogram looks continous. If you do the background correction in PixInsight and then save the linear image in 16 bit integer format for PS, you could get posterisation with a hard stretch.

But I'll check this first.

Edit: I've processed a few images lately where I needed to apply dbe several times to get a good background. I use the normalisation option in image correction. I haven't noticed any background posterisation in those images, even though the final background models had heavy posterisation on screen.

Edited by wimvb

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Great image Olly - really interesting bit of sky!

The dark neb top right is LDN792 I think. For the lower left HA region, I come up with Du27 (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Du+27&NbIdent=1) and Du29 (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Du+29&NbIdent=1) which are both references from http://articles.adsabs.harvard.edu/pdf/1976A%26AS...25...25D by Dubout-Crillon though I can't find the original catalogue (I think it was his own personal catalogue...) - the sizes/coords look reasonable matches.

BTW: Dubout-Crillon worked at the Observatoire de Haute-Provence when he published the paper above :)used a 5cm (!) refractor, 10 angstrom HA filter, with a FR reducing f5 to f1.25 (!!). Total of 20 plates, each 4hr exposures on pre-flashed plates... 

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12 minutes ago, coatesg said:

Total of 20 plates, each 4hr exposures on pre-flashed plates

7 hrs for the Heart and Soul nebulae. Some improvement since then.

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1 hour ago, coatesg said:

Great image Olly - really interesting bit of sky!

The dark neb top right is LDN792 I think. For the lower left HA region, I come up with Du27 (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Du+27&NbIdent=1) and Du29 (http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Du+29&NbIdent=1) which are both references from http://articles.adsabs.harvard.edu/pdf/1976A%26AS...25...25D by Dubout-Crillon though I can't find the original catalogue (I think it was his own personal catalogue...) - the sizes/coords look reasonable matches.

BTW: Dubout-Crillon worked at the Observatoire de Haute-Provence when he published the paper above :)used a 5cm (!) refractor, 10 angstrom HA filter, with a FR reducing f5 to f1.25 (!!). Total of 20 plates, each 4hr exposures on pre-flashed plates... 

Great, many thanks. LDN792 I'm sure it is, as you say. My internet is struggling with the other links up here in thunderland (!) but I'll follow them up.

Olly

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1 hour ago, wimvb said:

7 hrs for the Heart and Soul nebulae. Some improvement since then.

Actually quite impressive results, esp given reciprocity failure in film emulsion, and probably why they tried it at f1.25... But crikey, we have it easy now! :D

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20 minutes ago, coatesg said:

Olly, what were the exposure totals for each frame?

Cheers

The top panel was 6x10 minutes per colour and 7x20 mins Ha. The bottom panel was similar but also had a small amount of luminance. I don't remember just how much but not much at all.

In making the 2 panel I didn't start from scratch but stretched and processed the new top panel by eye till it was a reasonable match with the already-processed bottom one. I then just asked Registar to calibrate it and combine them, which it did almost seamlessly. One small region of visible join needed fixing simply by pasting the original panel over the one which the line showed and using it as a patch, feathered in.

Olly

PS Ha was added to red in Blend Mode Lighten and also used at very low opacity indeed as Luminance.

Edited by ollypenrice
PS
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Wow.  Talk about widefield.  I guess this is a taste of what awaits me with the FDQ at F3--wide.  (can't match this though--but it sure is inspiring)

Rodd

Edited by Rodd

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9 hours ago, wimvb said:

I'll do some more homework on this one. But what I've understood so far is that lut is a screen feature. However, if you have a 16 bit integer image, and stretch it hard in 16 bit, I would expect posterisation. But if the software converts the image to a higher bit depth, such as 32 bit floating point accuracy, there shouldn't be visible posterisation at any stretch. A 16 bit histogram looks spiked, while a 32 bit fp histogram looks continous. If you do the background correction in PixInsight and then save the linear image in 16 bit integer format for PS, you could get posterisation with a hard stretch.

But I'll check this first.

Edit: I've processed a few images lately where I needed to apply dbe several times to get a good background. I use the normalisation option in image correction. I haven't noticed any background posterisation in those images, even though the final background models had heavy posterisation on screen.

I can't find my copy of Keller's book at the moment (we're moving later this summer and have already started packing books; obviously too early).

But I was able to dig up an old thread on the PixInsight forum about this:

https://pixinsight.com/forum/index.php?topic=3641.msg25085#msg25085

It confirms what I wrote earlier: the DBE background model is smooth and in 32 bit floating point format. The STF stretch is a crude stretch that can cause posterisation on screen. One way to check that the DBE model is smooth, even if it looks posterised, is to use the pixel reader function in PI. move the cursor over the image, and see the pixel values change. Here are a few screen dumps of the effect. A mono image with DBE applied several times to reveal the posterisation in the background model. The pixel reader used over different parts of the image. Note how the pixel values continuously change, even if the image seems to have a constant intensity.

1802437884_Skrmklipp2018-06-1223_18_29.thumb.png.a4614b6e997778e012a1ccb370323cf5.png

1677663789_Skrmklipp2018-06-1223_18_42.thumb.png.ec8d82878839bc54cd19165834ada7e6.png

1696218769_Skrmklipp2018-06-1223_18_47.thumb.png.25c6fede963abf7a746d4548baa40343.png

201882134_Skrmklipp2018-06-1223_18_51.thumb.png.77fb634ad261ce9c2abd6c24ac01a714.png

Finally, the STF of the background model applied as a permanent stretch with histogram transformation: no posterisation.

1122214902_Skrmklipp2018-06-1223_23_37.thumb.png.f22a6769cfcad591fd89bc647c30857c.png

There was no posterisation in the image after applying dbe. But the image was 32 bit floating point to start with, and any posterisation in 16 bit integer mode can be concealed by noise.

Btw, here's some more information about the use of look up tables in the screen stretch function:

https://pixinsight.com/tutorials/24-bit-stf/

Edited by wimvb
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7 hours ago, wimvb said:

I can't find my copy of Keller's book at the moment (we're moving later this summer and have already started packing books; obviously too early).

But I was able to dig up an old thread on the PixInsight forum about this:

https://pixinsight.com/forum/index.php?topic=3641.msg25085#msg25085

It confirms what I wrote earlier: the DBE background model is smooth and in 32 bit floating point format. The STF stretch is a crude stretch that can cause posterisation on screen. One way to check that the DBE model is smooth, even if it looks posterised, is to use the pixel reader function in PI. move the cursor over the image, and see the pixel values change. Here are a few screen dumps of the effect. A mono image with DBE applied several times to reveal the posterisation in the background model. The pixel reader used over different parts of the image. Note how the pixel values continuously change, even if the image seems to have a constant intensity.

1802437884_Skrmklipp2018-06-1223_18_29.thumb.png.a4614b6e997778e012a1ccb370323cf5.png

1677663789_Skrmklipp2018-06-1223_18_42.thumb.png.ec8d82878839bc54cd19165834ada7e6.png

1696218769_Skrmklipp2018-06-1223_18_47.thumb.png.25c6fede963abf7a746d4548baa40343.png

201882134_Skrmklipp2018-06-1223_18_51.thumb.png.77fb634ad261ce9c2abd6c24ac01a714.png

Finally, the STF of the background model applied as a permanent stretch with histogram transformation: no posterisation.

1122214902_Skrmklipp2018-06-1223_23_37.thumb.png.f22a6769cfcad591fd89bc647c30857c.png

There was no posterisation in the image after applying dbe. But the image was 32 bit floating point to start with, and any posterisation in 16 bit integer mode can be concealed by noise.

Btw, here's some more information about the use of look up tables in the screen stretch function:

https://pixinsight.com/tutorials/24-bit-stf/

Thanks Vim, that's great. All is revealed.

Olly

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